CMOS image sensor, a photodiode thereof and a method of forming the same

Abstract

A photodiode (PD) of a complementary metal-oxide-semiconductor (CMOS) image sensor includes a top PD of a second type disposed in a first-type layer; and a bottom PD of the second type disposed in the first-type layer and below the top PD, the bottom PD including at least one sub-photodiode (sub-PD) of the second type connected to the top PD and at least one sub-well of the first type surrounded by the at least one sub-PD.

What is claimed is: 
     
       1. A complementary metal-oxide-semiconductor (CMOS) image sensor, comprising:
 a substrate; 
 a bottom PD layer disposed on the substrate, the bottom PD layer including at least one n-type sub-photodiode (sub-PD) disposed on the substrate; 
 at least one p-type sub-well included in the bottom PD layer and disposed on the substrate, the sub-well being surrounded by the at least one sub-PD; 
 an n-type top PD disposed on the bottom PD layer, the top PD directly connecting the at least one sub-PD physically and electrically; 
 a p-type transfer-gate channel disposed above the top PD; 
 a plurality of p-type pixel wells disposed on the substrate, adjacent pixel wells defining a PD region composed of the top PD and the bottom PD layer; and 
 an isolation region disposed above the substrate and between adjacent pixels; 
 wherein a top surface of the at least one sub-PD physically abuts a bottom surface of the top PD, and a bottom surface of the at least one sub-PD physically abuts a top surface of the substrate; and 
 a top surface of the at least one sub-well physically abuts the bottom surface of the top PD, and a bottom surface of the at least one sub-well physically abuts the top surface of the substrate. 
 
     
     
       2. The CMOS image sensor of  claim 1 , further comprising:
 a transfer gate disposed above and partially overlapping the transfer-gate channel; and 
 a floating diffusion (FD) node disposed in an upper portion of the pixel well, the FD node abutting the transfer-gate channel. 
 
     
     
       3. The CMOS image sensor of  claim 2 , wherein a first edge of the transfer gate partially overlaps the top PD, and an opposite second edge of the transfer gate partially overlaps the FD node. 
     
     
       4. The CMOS image sensor of  claim 1 , wherein the at least one sub-PD is ring-shaped. 
     
     
       5. The CMOS image sensor of  claim 1 , wherein the at least one sub-well is enclosed by a ring-shaped sub-PD. 
     
     
       6. The CMOS image sensor of  claim 1 , wherein the at least one sub-PD is symmetrical relative to a center of the bottom PD layer.